Photomasks are extensively used in the fabrication of integrated circuits on semiconductor wafers. Standard photomasks include a patterned absorbing or opaque film on a transparent substrate. A metal, such as chromium, having a thickness on the order of about 1000 .ANG. is often used as the opaque film. Nickel and aluminum are also used. A transparent material, such as quartz, is often used as the substrate, though materials such as glass and sapphire can also be used. Phase shift photomasks may include in addition to patterned chromium on quartz, means to change the phase of light near the chromium edge, such as an etched region in the quartz or a transparent layer on the quartz near a chromium edge. Phase shift photomasks can also include, instead of a totally opaque material, an absorbing material, also called an attenuator, that permits transmission of a small fraction of the light.
The fabrication of photomasks usually involves a blanket deposition of a thin film of the chromium opaque film on the quartz substrate. The chromium is then coated with photoresist, which is exposed with a high resolution technique, such as an electron beam, and developed to form the desired pattern in the resist. This pattern is then transferred into the chromium by etching, leaving opaque and transparent regions on the mask.
The photomask manufacturing process usually has imperfections, and defects are therefore frequently encountered during inspection of the photomasks. Defects are categorized as either "clear defects," or "opaque defects." Clear defects are regions that are designed to have the chromium but which actually do not have chromium. Opaque defects are regions that are designed to be clear of chromium but which actually do have chromium. FIG. 1 illustrates six of the most common defects seen on photolithography masks: FIG. 1a shows mask 20 having opaque bridge 21 between chromium lines 22a and 22b on quartz substrate 24; FIG. 1b shows opaque extension 26 extending from line 22a into transparent region 27 between lines 22a and 22b; FIG. 1c shows isolated opaque spot 28 in transparent region 27; FIG. 1d shows pinhole 30 in line 22a; FIG. 1e shows clear extension 32 into line 22a; and FIG. 1f shows clear bridge 34 extending across line 22a.
Masks having sufficiently large structures can be repaired using a technique described in U.S. Pat. 3,748,975, to Tarabocchia, in which photoresist is applied to the mask and exposed in a rectangular region including the defect to either open up a window for etching opaque defects away or to leave a region of tinted negative photoresist to fill in clear defects. However, for opaque and clear defects that are attached to metal lines normal process variations in the exposure, develop, and etch steps are too great to remove just the required amount of defect and provide a line having the desired shape within the tolerance required of present and future masks. For example, masks are currently being generated with lines having a width of 0.3um that typically have a tolerance in that width in the range 10% or, more preferably 5%. This precision is not achievable with standard photolithographic techniques, and the repair is likely either to leave unwanted material on the mask or to remove wanted material from the adjacent opaque region. Thus, present masks have been repaired using highly focused beams of photons or ions.
More specifically, opaque defect repair currently involves laser evaporation or ablation or focused ion beam (FIB) sputtering of the unwanted chromium in defect regions such as opaque bridge 21, opaque extension 26, and opaque spot 28. However, as with photolithographic processes, because the resolution of a laser is limited, if the opaque defect is connected to an adjacent chromium line (as in FIGS. 1a and 1b), laser ablation may damage that adjacent line, removing some wanted chromium from the line. In addition, because a great deal of thermal energy is transmitted with the laser beam, the laser ablation step not only melts and vaporizes the unwanted metal defect region, it also damages and removes a layer of quartz underlying and adjacent the opaque defect, producing roughness in the quartz, as described in a paper "Effect of Laser Mask Repair Induced Residue and Quartz Damage in Sub-half-micron DUV Wafer Process," y Pei-yang Yan, et al., Proceedings of the 15th Annual Symposium on Photomask Technology and Management, Sep. 20-22, 1995, SPIE Volume 2621, p. 158-166. This damaged region of the quartz is also responsible for reduced transmission and altered phase of transmitted light.
As an alternative to laser ablation, FIB offers a very controlled process for sputtering a small region of unwanted material. The ion beam can be focused to a much smaller size than the laser beam. In addition, the ion beam physically sputters material, transmitting very little thermal energy to the mask; thus, the quartz is not pitted. However, there are several problems that limit the use of FIB for mask repair. First, because masks are formed on quartz substrates, an insulating material, the ion beam rapidly charges the surface, and both the ability to aim subsequent ions and to use the ion beam to image the results is degraded. Second, while an opaque defect is being removed, quartz at the edge of the defect is attacked at the same rate, and the result is a "river bed" or trench of damaged quartz around the defect, the quartz in this region having altered transmission and phase. Third, the focused ion beam species is typically gallium, and gallium has been found implanted into the quartz when the opaque defect is removed, causing transmission losses. Fourth, the sputtering of material by the ion beam leads to ejection of material in all directions, and some of this ejected material comes to rest on adjacent edges as described in an article by J. M. E. Harper, et al., "Method for Improving Resolution of Focused Ion Beam Mask Repair Process," IBM TDB, n10a, March, 1991, p 174-176.
Clear defects can be repaired using a beam stimulated deposition process such as laser deposition or focused ion beam (FIB) induced deposition. These processes typically involve the decomposition of volatile organometallic complexes for which decomposition can be induced by the beam. Mask repair systems for laser deposition and FIB deposition have been commercially available for many years and the processes are well known in the art. However, inventors of the present invention have found that a thin halo of deposited material is found adjacent the laser repaired region. The halo is the result of deposition along the periphery of the laser spot where there is a low but non negligible light intensity. Thus, the edges of laser deposited material are difficult to control and trimming is usually required. This trimming step introduces the same kinds of problems and defects seen when removing other opaque defects.
Similarly, while FIB deposition to repair clear defects is more controllable than laser deposition, because the ion beam current profile also has a long tail which extends well beyond the nominal beam diameter, material is deposited in a relatively large area surrounding the intended deposit, as described in an article "Elimination of Excess Material during Focused Ion Beam Induced Deposition," by P. G. Blauner, IBM Technical Disclosure Bulletin, v 39, n1, January, 1996, p. 287-290. This peripheral film is a fraction of the thickness of the intended deposit, but often must be removed to avoid degradation of transmission in surrounding clear regions. Removal of the peripheral film, or "halo"requires an extra step which adds to the complexity and time required for the repair process and introduces another possibility for damaging the quartz or otherwise introducing defects. In addition, because the quartz substrate charges during ion beam processing, the ion beam can be deflected and the repair patch will therefore not be located where expected.
One proposed clear defect repair technique, described in U.S. Pat. 4,200,668, to Segal et al. (the '668 Patent), provides for repairing pin holes in the metal of a photomask by depositing resist on the mask, opening a window exclusively in the region of the pin hole by burning through the resist in that region with a laser, etching in the window to remove additional metal, thereby widening the pinhole and providing a more adhesive surface, depositing an opaque material on the entire surface, and then lifting off the opaque material on the resist, leaving it in the window adhering to the adjacent metal surrounding the pin hole. While this repair process is suitable for isolated pinhole defects, it is not adequate for clear defects at the edge of a metal line that required accurate reshaping and aligning, such as the clear extension and clear bridge of FIGS. 1e and 1f.
Thus, a better solution is needed that provides a more reliable method of correcting opaque and clear defects on photomasks, and this solution is provided by the following invention.